bb. 10-w-unit1 lect7-10 muscle posted handout
TRANSCRIPT
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Skeletal Muscle:Neuromuscular Junction
SarcomereSliding Filament Mechanism
Cross-bridge Cycle
Regulation by Ca2+
Excitation-Contraction CouplingUnit 1
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Muscle Converts Chemical Energy To Produce
Force & MovementPerformance:Movement, Speed,Strength, & Power
HEATInternal Movement:
Blood Flow, Digestion,etc
Heart Beat
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Figure 9.01
Figure 9-1- Muscle Types
Skeletal Cardiac Smooth
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Muscle contraction leads to:1. Support and movement of the skeleton
(skeletal muscle)
2. Generation of pressure in hollow cavities(cardiac muscle and smooth muscle)
3. Changes in the diameter of hollow tubes(smooth muscle)
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Muscle
Muscle fibers
Muscle fiber
MyofibrilSarcomere
Modified from McMahon,Muscles, Reflexes and LocomotionPrinceton University Press, 1984.
Structural Hierarchy of Skeletal Muscle
About half of the bodys mass is
composed of skeletal muscle
Most muscles linked to bones by
tendons
Skeletal muscles make up asystem of mechanical levers that
develop forces and via contraction
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Neuronal Control of Skeletal Muscle Contraction
Motor neuron cell body receivesinhibitory and excitatory inputs
from:
1. Afferent neurons
2. Spinal cord interneurons3. Cortex via descending tracts
Motor neuron output at
synapse is ALWAYS
excitatory to the muscle
fiber.
s
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Fig. 09.13aFigure 9-13a
ACh
ACh
ACh
ACh
ACh
1 Motor neuron innervates >1 Muscle fiber
1 Muscle fiber receives input from ONLY 1 motor neuron
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Fig. 09.14 The Neuromuscular Junction
The synapse between a motor neuron and a muscle fiber:
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Fig. 09.15
Figure 9-15
Neuromuscular
Junction-
Events leading
to AP in musclefiber
= end plate potential(EPP)
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Relationship between AP in motor neuron and EPP (end plate potential)
1. EPP is always excitatory (depolarizing)
2. EPP is graded, but is always above threshold and generates APs in the
muscle membrane
3. The APs propagate along the membrane and into the T-tubules,
causing Ca2+ release from the SR and cross-bridge cycling
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Figure 9-10 Relationship between muscle AP and muscle contraction
(Twitch)
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Fig. 09.02
Figure 9-2
Structure of Skeletal Muscle
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Fig. 09.03
Figure 9-3 The Sarcomere
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THE SARCOMERE
ACTIN
Myosin
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Fig. 09.07
Figure 9-7 Thick and Thin Filaments
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Figure 9-6 Sliding Filament Mechanism
Contraction:Activation of force generating sites.
May or may not be associated with shortening
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Fig. 09.05
Figure 9-5
Sliding Filament
Mechanism
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How striatedmuscle works:The Sliding Filament Model
thickandthin filamentsinterdigitate
filaments sliderelativeto each other
filamentlengthdoesnotchange
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From Vander, Sherman, Luciano
Human Physiology, McGraw-Hill.
Antiparallel arrangement of myosin heads pull the Z-lines
towards one another causing the sarcomere to shorten
(animation)
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Figure 9-8
The Cross Bridge Cycle- Mechanism of Force Generation in Muscle
1. Attach
2. Power Stroke
Rate limiting step = release ofPi
3. Detachment
4. Energize
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Figure 9-8
The Cross Bridge Cycle- Mechanism of Force Generation in Muscle
1. Attach
2. Power Stroke
Rate limiting step = release ofPi
3. Detachment
4. Energize
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Figure 9-8
The Cross Bridge Cycle- Mechanism of Force Generation in Muscle
1. Attach
2. Power Stroke
Rate limiting step = release ofPi
3. Detach
4. Energize
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Figure 9-8
The Cross Bridge Cycle- Mechanism of Force Generation in Muscle
1. Attach
2. Power Stroke
Rate limiting step = release ofPi
3. Detach
4. Energize
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Fig. 09.09
Figure 9-9
Ca
2+
Activation ofCross-Bridge Cycling
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Regulation of contraction: Ca2+, Troponin, & Tropomyosin
(Animation)
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Figure 9-11a Sarcoplasmic Reticulum, T-Tubules and Myofibrils
Source of cytosolic Ca2+ for skeletal muscle activation is the
sarcoplasmic reticulum (SR)
Ca2+ release is stimulated by AP propagating down T-tubule membrane
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Figure 9-11b SR, T-Tubules and Myofibrils within a skeletal muscle fiber
Ended here 9/25
S i i G i
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Sarcoplasmic Reticulum RyRGatingfrom Germann and Stanfield,Principles of Human Physiology; Pearson, Benjain Cummings 2008
Figure 12.10
DHP = dihydropyridine
receptor = voltage sensor in
T-tubule membrane
Ryanodine receptor: Ca2+
channel in the SRmembrane. Interacts
with DHP receptor, opens
and releases Ca2+ to cytosol
resulting in contraction
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Fig. 09.12
Figure 9-12
Excitation-Contraction
Coupling in
Skeletal Muscle
SERCA
SERCA: Ca2+ ATPase
on SR membrane thatpumps Ca2+ into SR,
thereby lowering
cytosolic [Ca2+]
resulting in relaxation
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1
Functions of ATP in Muscle
Contraction and Relaxation1. Hydrolysis of ATP by Myosin
Provides energy for force generation and cross-bridge movement.
2. Binding of ATP to Myosin
Dissociates myosin cross-bridge from actinallowing cross-bridge cycling and preventingrigor mortis.
3. Hydrolysis of ATP by CaATPase
Provides energy to pump Ca 2+ into SR producingmuscle relaxation.
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2
Figure 9-10 Relationship between muscle AP and muscle contraction
(Twitch)
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Problem 1
What would happen if skeletal muscle fibers
were stimulated to contract and then during
contraction the [ATP] was decreased
significantly?
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Muscle Contraction
The action potential propagating down the axon leads to a ___________ of theaxon terminal membrane, which leads to the opening of ________channels in the
terminal membrane and diffusion of ______________the axon terminal. This
causes release of ___________ into the synaptic cleft, which diffuses across the
cleft and binds to receptors on the_______________ of the muscle cell. Binding
leads to opening of an ion channel permeable to___________, and the diffusion of
more_______ into the cell than ________ out of the cell and therefore __________of the membrane. This __________ of the membrane causes the opening
of_____________ and firing of an ____________ in the muscle fiber membrane.
The _____________ is propagated along the muscle fiber membrane and to the
interior of the fiber via_______________, and leads to release of _________ fromthe_________________. The_______ binds to _____________causing
_______________ to shift so that __________ binding sites are revealed on
___________. The__________ bind to __________ and__________ cycling
occurs. Relaxation occurs because _________ is returned to the ___________ by
the activity of the ___________ in the ______ membrane.
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What you should have learned:
Skeletal muscle fiber, myofibril andarrangement of filaments in striated muscle
Sarcomere
Thick filament/myosin Thin filament/actin, troponin, tropomyosin
Cross bridge cycle
Roles of ATP in contraction and relaxation Excitation-Contraction coupling
Events at NMJ (neuromuscular junction)
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Mechanics of Single Fiber Contraction
Load- force exerted on muscle by the weight of anobject
Tension- force exerted on an object by a contractingmuscle
Twitch- mechanical response of a single musclefiber to a single action potential
Fi 9 16
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Fig. 9-16
Isometric and Isotonic Contractions
Velocity of contraction = Distance shortened/time = x/t
Isometric:
Fixed Length
Isotonic:Allowed to
shortenFixed Load
x
t
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Lengthening Contraction
(also referred to as eccentric contraction)
Load > Tension
Muscle is pulled to a longer length
Not an active process
Example- sitting down, lowering an object
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3 Key Relationships
Load-Velocity (Force-Velocity)
Isotonic Contractions
Velocity varies with load Frequency-Tension (Force-Frequency)
Isometric Contractions
Force varies with frequency of stimulations
Length-Tension Isometric Contractions
Force varies with starting sarcomere length
i 9 17 LOAD VELOCITY RELATIONSHIP
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Figure 9-17 LOAD-VELOCITY RELATIONSHIP
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Figure 9-18 LOAD-VELOCITY RELATIONSHIP
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Figure 9-19
FORCE-FREQUENCY
RELATIONSHIP
Frequency of APs in fiber membrane
1 AP/200 ms = 5 AP/sec
Frequency of APs in fiber membrane
1 AP/75 ms = 13 AP/sec
Frequency of APs in fiber membrane
1 AP/10 ms = 100 AP/sec
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Figure 9-20 FORCE-FREQUENCY RELATIONSHIP
frequency = 10 AP/sec
frequency = 100 AP/sec
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Whats the mechanism behind the
force-frequency relationship? Why isnt the twitch force maximal after a
single AP?
What change occurs with increased APfrequency that causes force generation toincrease?
What limits the maximum force that can begenerated with fused tetanus?
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Figure 9-21 LENGTH-TENSIONRELATIONSHIP
LENGTH TENSION = Longer sarcomere lengths
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LENGTH-TENSION
RELATIONSHIP
4. Overlap of thin filaments
interferes with myosin-actin
interactions.
Shorter Sarcomere Lengths
= Longer sarcomere lengths
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Metabolic Pathways
similar to Figure 9-22
Figure 12.11Germannand Stanfield, Principles ofHuman Physiology, Pearson
BenjaminCummings, 2008
During skeletal muscle
contraction, the primary
metabolic pathway used to
provide ATP depends onintensity of exercise and fiber
types recruited
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Creatine Phosphate:
First available source of ATP and rapidly depleted (8-10 sec)
Utilized at the onset of any activity
Provides time for other metabolic pathways to be turned on
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Creatine Supplementation
Decreases endogenous creatine production Small increase in muscle creatine levels (15-20%) that maxes
out in a couple of days
Creatine stimulates protein synthesis, so increase in musclemass if supplementation + exercise compared to just exercise
Enhance performance in high intensity tasks lasting < 30seconds, but not in moderate intensity, longer durationactivities
Side effects of short term supplementation Weight gain
Muscle cramps GI difficulties
Dehydration
Heat intolerance
Effects of long term Creatine supplementation not adequatelystudied yet.
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Oxidative Phosphorylation
P
rovides most of the ATP
for moderateintensity exercise
Occurs in the Mitochondria
Requires Oxygen Primary Fuel Sources:
Glycogen for first 5-10 minutes
Blood glucose and fatty acids for next 30 min.
Fatty Acids after35-40 minutes
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Glycolysis Makes significant amount of the ATP during high intensity
exercise High intensity = intensity exceeding 70% of maximal ATP
breakdown rate
Glucose Pyruvic acid+ 2 ATP Lactic acid + 2 ATP
Can produce ATP
in absence of oxygen During high intensity activity, adequate blood flow and O2 delivery arelimiting factors.
Can produce large amounts of ATP when enough enzymesand substrates are available
Provides ATP for about 1.3-1.6 minutes of maximal activity Fuel Sources
Glycogen
Blood Glucose
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Primary Energy Systems Used for SportsModified from Table 84-1, Textbook of Medical Physiology, Guyton and Hall, Elsevier Saunders, 2006
CP CP +
Glycolysis
Glycolysis Glycolysis
+ OxidativePhosph.
Oxidative
Phosph.
100 m dash 200 m dash 400 m dash 800 m dash 10,000 m
skate
Jumping Basketball 100 m swim 200 m swim Cross
countryskiing
Weight
lifting
Ice hockey
dashes
Tennis 1500 m
skate
Marathon
Diving Soccer 2000 mrowing Jogging
Football
dashes
1500 m run
400 m swim
Skeletal Muscle has 3 predominant fiber types
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Slow fibers (Type I) have a slow myosin ATPase, and so split ATP
at a slower rate. Fatigue more slowly.
Fast fibers (Type IIa and IIb) have a fast myosin ATPase and so
split ATP at a faster rate. Fatigue more quickly.
Skeletal Muscle has 3 predominant fiber typesA whole muscle is a mix of the 3 fiber types
Type I Type IIa Type IIb
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Oxidative fibers are specialized to use primarily oxidativephosphorylation to produce ATP.
Glycolytic fibers are specialized to use primarily glycolysis to
produce ATP
Figur
e 9-25
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Figure 9-25
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Muscle Fatigue
Muscle Fiber Fatigue- decline in muscletension as a result of previous contractile
activity (weakness that results from activity)
Two Types:
High Frequency
Low Frequency
It is NOT due to significant ATP depletion (why
not?)
So what does cause it?
T pes of Fatig e
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Types ofFatigue High Frequency Fatigue
from high intensity, short duration exercise
Continuous stimulation
Onset is rapid
Recovery is rapid
Caused by increased Pi, conduction failure, pH decrease
Low Frequency Fatigue from low intensity, long duration exercise
Cyclical contractions and relaxations
Onset is slower
Recovery period is longer (replenish glycogen, replace damagedproteins)
Major contributors are probably depletion of glycogen, low blood
glucose, dehydration (electrolyte imbalances, volume depletion), and
decreased pH
High Intensity Fatigue: why does elevated P
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High Intensity Fatigue: why does elevated Picontribute to fatigue ?
Cross bridge cycling will slow as Pi
levels increasewhy?
Hi h I t it F ti C d ti F il ?
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High Intensity Fatigue: Conduction Failure?
Think about the T-tubular space andion changes during action potentials.
Why/how, with repeated stimulation,
could no action potentials be
produced?
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All the muscle fibers in a single
motor unit are of the same type.
Whole muscle is a mixture of
motor units.
The relative proportion of the
3 fiber types determines
muscles maximumcontraction speed,
strength and
ability to resist fatigue.
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Recruitment of Motor Units
Figure 12.19
Figure 11-30
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Figure 11 30
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Size Principle Size of motor unit relates to size of fibers and size of neurons
Small motor units small muscle fibers (ie. slow oxidative)
Small muscle fibers usually innervated by motor neuron with small cell bodiesand small diameter axons
Larger motor units
large muscle fibers (ie. fast glycolytic)
Large muscle fibers usually innervated by motor neuron with large cell bodiesand large diameter axon
Order of motor unit recruitment relates to size of motor units Large neurons harder to depolarize to threshold (need greater synaptic
input)
Small neurons activated at low input, so small fibers and small motor unitsactivated first
Large neurons activated at high input, so large fibers and large motor unitsactivated last
Good because small fibers more resistant to fatigue than large fibers
Motor Unit Size Principle
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Motor Unit Size Principle
Figure 12.20Germannand Stanfield,
Principlesof Human
Physiology, Pearson
BenjaminCummings, 2008
Smaller motor
neurons (cell
bodies and
axons) innervatesmaller motor
units and slow
fibers and are
activated first;
so X, activatedbefore Y,
activated before
Z
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Skeletal Muscle
Exercise (training) can change the size and strength of muscle as
well as its metabolic capacity (oxidative or glycolytic), but does
not significantly alter the speed of contraction (fast or slow
myosin).
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Muscleisplastic!
Meaningitcan Adaptto differing work
demandsplaced onit.
Increasemuscle use:hypertrophy,
increasesarcomeresinparallel
Decreasemuscle use atrophy,decrease
musclemass,losesarcomeresinparallel
Effects of Exercise and TrainingEffects of Exercise and Training
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Strengthtraining ( weightlifting )
-hypertrophy of muscle,all fibertypesrecruited willhypertrophy
Endurancetraining ( marathon )
- littlehypertrophy
- majorbiochemicaladaptationsin muscle
- increasecapillariespermuscle fiber
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What about fiber type conversions?What about fiber type conversions?
- controversial, withendurancetrainingthere
isevidence of typeIIb (fastglycolytic)
switchingto typeIIa (fast oxidative)
- Hardto getgooddatainhumans
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Type I Type IIa Type IIb
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Distribution of Fiber Types
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Distribution of Fiber TypesModified from Table 1.3, p. 45, Physiology of Sport and Exercise, 1999.
ATHLETEATHLETE GENDERGENDER MUSCLEMUSCLE %% SLOWSLOW % FAST% FAST
Sprinters Male Gastroc. 24 76
Female Gastroc. 27 73
DistanceRunners
Male Gastroc.7
9 21
Female Gastroc. 69 31
Weight-
lifters
Male Gastroc. 44 56
Nonathlete Male V.Lat. 47 53
Female Gastroc. 52 48
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Predict the Adaptations to
Aerobic Training Muscle fiber type
Capillary supply
Myoglobin content
Mitochondria
Oxidative enzymes
What should you have learned?
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What should you have learned?
Isometric, isotonic and lengthening contractions
Load-Velocity Relationship
Frequency-Tension (Force-Frequency) Relationship
Length Tension Relationship Skeletal muscle energy
metabolism (sources of ATP) Types and causes of fatigue
Fiber types- similarities and differences
Whole muscle contraction and recruitment How does muscle adapt to exercise?
Understand the problem answers
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Smooth Muscle
Unit 1
S th M l R l
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Smooth Muscle Roles
Surrounding a hollow organ or tube: contractiongenerates a pressure to propel contents (increase
flow)
GI Tract
Bladder
Uterus
Surrounding tubes: Contraction changes diameter to
regulate flow (increase resistance to flow) GI Tract
Blood vessels
Airways
Figure 9 33
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Figure 9-33
Arrangement of
thick and thin filaments
in smooth muscle
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Comparison of smooth and
skeletal muscle Maximum tension per unit cross-sectional
area is similar between smooth and skeletal
Smooth muscle also demonstrates a
Length-Tension relationship, but...
Smooth muscle can develop tension over a larger
range of muscle lengths than can skeletal
Figure 9-7 Thick and Thin Filaments
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Fig. 09.07
g
Smooth Muscle- phosphorylation of
myosin light chain regulates
myosins binding to actin
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Fig. 09.34
Figure 9-34 Activation of smooth muscle contraction by Ca2+
Cross Bridge Cycle in Smooth Muscle
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Increased Ca2+-Calmodulin
Increased MLCK/MLC Phosphatase ratio
PO4
PO4 + ADP
PO4 + ADP
PO4 ADP
PO4
2. Power stroke
PO4
ATP3. Detachment4. Energize
Decreased Ca2+
-Calmodulin
Decreased MLCK/MLC Phosphatase ratio
C oss dge Cyc e S oo usc e
Other than attachment step, cycle is the same as skeletal
ATP
Smooth Muscle Contraction and
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Relaxation Contraction: Ca2+-CM complex activates MLCK,
MLCK activity > MLC phosphatase activity, MLCphosphorylated and cross-bridge cycling occurs
Rate of ATP Utilization:
Lower than in skeletal muscle
Shortening velocity is slower than skeletal
Very fatigue resistant
Relaxation: Ca2+ is removed from the cytosol by
Ca2+ ATPase on plasma membrane
Ca2+ ATPase on SR membrane
Ca2+-Na+ exchanger on the plasma membrane
MLC phosphatase activity > MLCK activityp cross
bridge cycling stops
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Fig. 09.35Figure 9-35
Comparison ofSmooth and Skeletal
Ca2+ Activation
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Multiple Activating
Signals for Smooth
Muscle
Chemical Messengers such asHormones
Paracrine factors
Neurotransmitters
Local metabolites
These messengers may causecontraction or relaxation
(excite or inhibit).
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Smooth muscle actionpotential depolarization
is due to Ca2+influx
Figure 9-36a Spontaneous Electrical Activity (PacemakerPotential)
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A
B
C
A: Ca2+ dependant K+ channels close and the membrane depolarizes
B: Voltage-gated Ca2+ channels open and AP occurs, cytosolic Ca2+ rises
C: Ca2+ dependant K+ channels open and membrane hyperpolarizes
D: V-gated Ca2+ channels close and cytosolic Ca2+ decreases
D
No stable resting Vm
in pacemaker cells.
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Figure
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90
g
9.38
Single Unit Smooth Muscle
Characteristics
1. Connected by gap junctions
2. Synchronous activity
3. Contain pacemaker cells
4. Activity altered by inputs (ie. changing
autonomic activity) to pacemaker cells
5. Contraction can be initiated by stretch
6. Examples: GI, uterine and small diameterblood vessel smooth muscle.
Figure
9 37
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9.37
Multiunit Smooth Muscle CharacteristicsFew or no gap junctions
1. Activity is not synchronous2. No pacemaker cells
3. Richly innervated throughout the muscle
4. Not responsive to stretch
5. Contractions often do not require APs in the membrane
6. Examples: Airway and large artery smooth muscle
Comparison of Muscle Types
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Characteristic Skeletal SmoothThick & Thin filaments
Sarcomeres-banding
Transverse tubules
Sarcoplasmic Reticulum
Gap Junctions
Ca2+ source
Site of Ca2+ regulation
Spontaneous APs
Effect of nerve input
Effect of hormonal input
Stretch causescontraction
What should you have learned?
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y
Similarities and differences between skeletal
and smooth muscle
Arrangement of thick and thin filaments, dense
bodies, length-tension relationship
Cross-bridge activation
Sources of cytosolic Ca2+ and mechanisms
regulating cytosolic [Ca2+]
Neural input to smooth muscle, spontaneousAPs, pacemaker potentials
Single unit vs. Multi unit smooth muscle